The fabrication of semiconductor devices typically involves a series of fabrication steps or process operations to produce the desired structures and circuits in the semiconductor device. Depending on the types of structures and circuits desired on the semiconductor device, varying sequences of process steps are used in the production of the device.
Of course, production is less costly in most cases if the number of processing steps needed to arrive at a finished semiconductor device can be minimized. A minimization of process steps in most cases also serves to decrease the probability of errors occurring in the manufacture of the device. Once a series of process steps has been developed for manufacturing a certain device, it is economically costly to revise those steps. Such revision frequently requires changes in the processing operations in the manufacturing facility, possibly requiring new equipment, changes in existing equipment, alterations in production flow and operation sequences as well as other changes. Additionally, any change in the fabrication of the semiconductor device must be studied carefully and also refined to avoid possible adverse affects to the device arising from the changed processes.
There are several general types of semiconductor devices, each having its own field of fabrication technology. One general type of semiconductor device comprises an integration of CMOS (complimentary metal oxide semiconductor) structures and double polysilicon FAMOS (floating gate avalanche metal oxide semiconductor) structures in one semiconductor device referred to herein as a CMOS/FAMOS device. It should be understood that CMOS devices comprise integrations of NMOS (N channel field effect transistors) and PMOS (P channel field effect transistors) devices. A particular series of process steps or fabrication technology has been developed for manufacturing the CMOS/FAMOS semiconductor devices. For the sake of clarity, this fabrication technology is referred to herein as double polysilicon CMOS technology, or simply CMOS technology.
Another general type of semiconductor device comprises bipolar transistor structures in the semiconductor device. A particular fabrication technology, referred to herein as bipolar technology, has been developed for manufacturing bipolar transistors on semiconductor devices.
With the development of new semiconductor circuit designs, including those for analog type circuits, it has become desirable to provide, on the CMOS/FAMOS type semiconductor devices, transistors having a high transconductance. Such transistors would provide a high drive capability for the circuits of the device and, consequently, improve the circuit speed of the device. However, for a given transistor structure size, CMOS structures provide a relatively low drive capability in comparison with that of bipolar transistors. Accordingly, it is desirable to integrate bipolar transistors, due to their high transconductivity and consequent high drive capability, into semiconductor devices which embody CMOS/FAMOS structures.
However, the conventional fabrication technology for bipolar transistor structures and that of typical CMOS/FAMOS structures do not coincide. As a result, when it is desired to integrate bipolar transistors with CMOS/FAMOS structures on a semiconductor device, separate fabrication steps are necessary to form the bipolar devices and the CMOS/FAMOS device, i.e., a series of fabrication steps must be carried out to form the bipolar transistor and a separate series of steps carried out to form the CMOS/FAMOS structures. Consequences of these increased steps are increased manufacturing costs as well as decreased reliability of manufacture due to the greater number of steps required for the manufacturing process. As a result, the need exists for a process to fabricate bipolar transistor devices on a semiconductor device simultaneously with fabrication of CMOS/FAMOS structures on the same device using conventional CMOS fabrication technology and without the introduction of separate and additional processing steps to fabricate the bipolar devices.